A Substantial linear red shift in the band gap in heavily copper doped zinc oxide thin films produced by co-sputtering

Zinc oxide is a wide band gap semiconductor that has found potential applications in ultraviolet optoelectronics. Its utilization in the visible range necessitates its energy gap be reduced and tunable. Here, this is achieved by doping ZnO thin films with copper via a co-sputtering technique, where the ZnO was radio-frequency sputtered and the copper was DC-sputtered. The copper concentration was controlled through the DC power applied to the copper target, where concentrations up to 51 at.% were attained indicating heavy doping. The resulting films exhibited large absorption in the visible range with a considerable red shift in the band gap approaching a value of 0.57 eV compared to pure ZnO.